Vectors and methods for tissue specific synthesis of proteins in eggs of transgenic hens

ABSTRACT

Vectors and methods are provided for introducing genetic material into cells of a chicken or other avian species. More particularly, vectors and methods are provided for transferring a transgene to an embryonic chicken cell, so as to create a transgenic hen wherein the transgene is expressed in the hen&#39;s oviduct and the transgene product is secreted in the hen&#39;s eggs and/or those of her offspring. In a preferred embodiment, the transgene product is secreted in the egg white.

RELATED APPLICATIONS

This application is a continuation-in-part of U.S. Application Ser. No.60/019,641, entitled “Vector For Expression Of Proteins Into Eggs OfTransgenic Hens,” filed Jun. 12, 1996.

FIELD OF THE INVENTION

The present invention relates generally to vectors and methods forintroducing genetic material into an embryo of a chicken and other avianspecies and, more particularly, to vectors and methods for transferringa gene of interest to an embryonic chicken cell, so as to create atransgenic hen having the gene of interest expressed in the hen'soviduct and the gene product secreted in the hen's eggs and/or those ofher offspring.

BACKGROUND OF THE INVENTION

Since the development of recombinant DNA technology some twenty-fiveyears ago, the prospect of producing proteins on a large scale, ratherthan extracting them from tissue where they are naturally expressed, hasbecome a reality. In particular, over the last two decades, progress inthe development of expression vectors has led to the production ofthousands of recombinant proteins on a laboratory scale. Production ofcommercial quantities of recombinant proteins requires often difficultand expensive scaling up procedures, but has nonetheless also beensuccessful. In addition, transgenic animals including mice, rabbits,pigs, sheep, goats and cows have been engineered to produce humanpharmaceuticals in their tissues or secretions. Houdebine, L. M., J.Biotechnology 34:269-287 (1994).

Although egg white is thought to be an excellent host for recombinantprotein production, preparing transgenic avians has proven to betechnically difficult due in large part to problems involved inmanipulating the chicken embryo. When oviposition occurs, the embryo hasalready reached a stage corresponding to a mammalian late blastula orearly gastrula. Genetic manipulation of the embryo during earlierdevelopment requires reintroduction to the female or in vitro culture,both difficult procedures. Houdebine, L. M., J. Biotechnology 34:269-287(1994). Despite these difficulties, transgenic chickens have beenproduced that are resistant to infection by avian leukosis (Crittendenand Salter, “Transgenic Livestock Models In Medicine And Agriculture”pp. 73-87 (Wiley-Liss (1990))), or have high levels of circulatinggrowth hormone. Bosselman, R. A., et al., Science 243:533-535 (1989).

Four general methods for generating transgenic avians have beenreported. One method involves excision of a developing egg from theoviduct, microinjection of DNA near the blastoderm, and in vitro cultureof the manipulated embryo in solution and surrogate shells. Love, J., etal., Biotechnology 12:60-63 (1994). A second method requires the cultureand transfection of primordial germ cells, with subsequenttransplantation into an irradiated recipient near the same stage ofdevelopment as the donor. Carsience et al., Development 117:669-675(1993); Etches et al., Poultry Science 72:882-889 (1993). Althoughtechnically very demanding, these two approaches are attractive becauselarge pieces of DNA can be transferred.

A third method involves blind injection of replication competentretrovirus with a needle near the blastoderm of a newly laid egg.Petropoulos, C. J., et al., J. Virol. 65:3728-3737 (1991). Although thismethod is the simplest, it is also limited in that the DNA to betransferred must be approximately 2 kb or less in size and, the methodresults in viremic hens which shed infective recombinant retrovirus.Petropoulos, C. J., et al., J. Virol. 66(6):3391-3397 (1992).

The fourth method involves a replication-defective retroviral vectorsystem (see, e.g., U.S. Pat. Nos. 5,162,215 and 4,650,764, herebyincorporated by reference). One of these systems (Watanabe and Temin,Mol. Cell. Biol. 3(12):2241-2249 (1983)) has been derived from thereticuloendotheliosis virus type A (REV-A). Sevoian et al., Avian Dis.8:336-347 (1964). Replication-defective retroviral vectors derived fromthe REV-A virus are based on the helper cell line C3 (Watanabe andTemin, Mol. Cell. Biol. 3(12):2241-2249 (1983)) which contains thecomponents of a packaging defective helper provirus. The derivation ofthe C3 helping line and several replication-defective retroviral vectorshave been described in detail in U.S. Pat. No. 4,650,764 and Watanabeand Temin, Mol. Cell. Biol. 3(12):2241-2249 (1983). This method is moretechnically demanding than the replication competent technique in thatthe blastoderm must be exposed, and microinjection equipment must beused. Bosselman, R. A., et al., Science 243:533-535 (1989). Nonetheless,it results in transgenic hens free of replication competent retrovirus,and can transfer DNA as large as 8 kb in size.

Tissue specific expression of a foreign gene in a transgenic chicken wasachieved using the replication competent retrovirus technique.Petropoulos, C. J., et al, J. Virol. 66(6):3391-3397 (1992). Areplication competent retrovirus was used to deliver the reporter genechloramphenicol acetyl transferase (CAT), driven by a muscle specificpromoter, a action, to skeletal muscle. Tissue specific expression of arecombinant protein in the egg of a transgenic avian has not yet beensuccessful.

It would thus be desirable to provide a vehicle and method fortransferring a gene to an embryonic chicken cell (or other avianspecies) so as to create a transgenic hen wherein the gene is expressedin a tissue specific manner. It would also be desirable to provide avehicle and method for transferring a gene to an embryonic chicken cell,wherein the gene is expressed in the hen's oviduct and secretion of thegene product is in the hen's eggs. It would also be desirable to providea vehicle and method for transferring a gene to an embryonic chickencell, wherein the gene is expressed in the hen's oviduct and secretionof the gene product is in the hen's eggs without compromise to the hen'shealth and the health of other birds in contact with her.

SUMMARY OF THE INVENTION

Vectors and methods are provided for introducing genetic material intocells of a chicken or other avian species. More particularly, vectorsand methods are provided for transferring a transgene to an embryonicchicken cell, so as to create a transgenic hen wherein the transgene isexpressed in the hen's oviduct and the transgene product is secreted inthe hen's eggs and/or those of her offspring. In a preferred embodiment,the transgene product is secreted in the egg white.

In one embodiment, the vector comprises a portion of a retroviralgenome, capable of transfecting a cell and incapable of replication,i.e., a replication-defective retroviral vector. The vector furthercomprises a transgene, operatively-linked to appropriate controlelements such that the transgene may be expressed in a tissue specificmanner. In one embodiment, the control elements include an enhancedpromoter directing the expression of the transgene in the oviduct, anuntranslated region 5′ to the structural gene (coding region) ofappropriate length and sequence to promote efficient translation, and asignal sequence directing the secretion of the transgene product in theegg white. In this embodiment, the promoter may be chosen, withoutlimitation, from the group consisting of ovalbumin, lysozyme, conalbuminand ovomucoid promoters, and combinations thereof. In anotherembodiment, the control sequences include a promoter directing theexpression of the transgene in the liver and a signal sequence directingthe uptake and secretion of the transgene product into the egg yolk. Inthis embodiment, the promoter may be chosen, without limitation, fromthe group consisting of vitellogenin and apolipoprotein A promoters, andcombinations thereof.

The vectors of the present invention may be used in producing transgenicavians, particularly chickens, by methods known to those skilled in theart, such as the four methods described above (see, Background Of TheInvention). For example, as described in U.S. Pat. No. 5,162,215, hereinincorporated by reference, the vectors may be used to introduce anucleic acid sequence, e.g., a gene, into germ cells and stem cells ofan embryo of a chicken. In one embodiment, the vector is microinjectedin a newly laid chicken egg, in close proximity to (e.g, directlybeneath) the blastoderm. The egg is then sealed and incubated until thechicken is hatched from the egg. The transgenic chicken is then testedfor expression of the transgene and if positive and the chicken isfemale (hen), the eggs of the chicken are harvested and the protein isisolated by methods known to those skilled in the art. If the chicken ismale (rooster), it can be bred to produce a female transgenic chickenwhose eggs may then be harvested. Transgenic avians and eggs, as well asmethods of making transgenic avians and eggs, are thus provided.

Other features and advantages of the present invention will becomeapparent from the following description and appended claims, taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present invention will become apparent toone skilled in the art by reading the following specification andsubjoined claims and by referencing the following drawings in which:

FIG. 1 is a schematic illustrating the production of the vectors of thepresent invention and methods of using same to produce transgenicchickens;

FIG. 2 is a schematic illustrating a preferred vector of the presentinvention;

FIG. 3 is a schematic illustrating construction of the retroviral andexpression vectors of the present invention;

FIG. 4 is a schematic illustrating the construction of intermediate #1,pOVSV;

FIG. 5 is a schematic showing the construction of intermediate #2,pSigI;

FIG. 6 is a schematic illustrating the construction of intermediate #3,pSigPCR;

FIG. 7 is a schematic showing the construction of intermediate #4, pUTR;

FIG. 8 is a schematic illustrating the construction of intermediate #5,pUTRAN;

FIG. 9 is a schematic showing the construction of intermediate #6, pERE1;

FIG. 10 is a schematic showing the construction of intermediate #7, pERE(note in FIG. 10 that arrows are for orientation of the ERE sequencewithin the oligonucleotides, not the oligonucleotide itself);

FIG. 11 is a schematic illustrating the modified proviral vector;

FIG. 12 is a schematic illustrating the modification of the 3′ end ofthe hygromycin B phosphotransferase gene; and

FIG. 13 is a schematic showing the modification of the N-terminus ofhygromycin B phosphotransferase gene.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Vectors and methods are provided for introducing genetic material intocells of a chicken or other avian species. More particularly, vectorsand methods are provided for transferring a transgene to embryonicchicken cells, so as to create a transgenic hen wherein the transgene isexpressed in the hen's oviduct and the transgene product is secreted inthe hen's eggs and/or those of her offspring. FIG. 1 is a schematicillustrating the methods of the present invention, including vectorproduction and use to produce a transgenic chicken.

In one embodiment, the vector comprises a portion of a retroviralgenome, capable of transfecting a cell and incapable of replication,i.e., a replication-defective retroviral vector. Replication-defectiveretroviral vectors derived from the REV-A virus are preferred. Thevector further comprises a gene of interest also referred to herein as atransgene, operatively-linked to appropriate control elements such thatthe transgene product may be synthesized in a tissue specific manner.

A schematic of a preferred expression vector of the present invention isset forth in FIG. 2. It will be appreciated that the 3 kbβ-galactosidase gene shown in FIG. 2 is merely a reporter gene and isreplaced with any transgene(s) or fragment thereof. For example a genewhich encodes a blood clotting protein such as fvIII, may be employed.The transgene product or protein, is secreted in the egg and thenisolated. Once purified, the protein may be used in pharmaceuticalapplications such as in the treatment of hemophilia. Other preferredgenes include, without limitation, the genes encoding blood proteinsincluding human serum albumin and α 1-antitrypsin, hematopoietic growthfactors including erythropoietin, and lymphopoietic growth factors suchas granulocyte colony stimulating factors. Genes encoding industrialproteins such as α-amylase and glucose isomerase may also be employed.Moreover, genes encoding antibodies and immunoreactive portions thereof,may also be included in the vectors of the present invention (see, e.g.,Lilley, et al., J. Immunol. Meth. 171:211-226 (1994) and Davis et al.,Biotechnol. 9:165-169 (1991), herein incorporated by reference).

The gene, or a fragment of the gene, to be transferred may be producedand purified by any of several methods well known in the art. Thus, agene can be produced synthetically, or by treating mRNA derived from thetranscription of the gene with a reverse transcriptase so as to producea cDNA version of the gene, or by the direct isolation of the gene froma genomic bank or from other sources.

Control elements which flank the transgene include promoters andenhancers, UTRs and signal sequence(s), that allow tissue specificexpression of the transgene. In one embodiment, the promoter directsexpression of the transgene in the oviduct of the transgenic avian. Apreferred promoter of the present invention is chosen from the groupconsisting of ovalbumin, lysozyme, conalbumin and ovomucoid promoters,and combinations thereof. Signal sequences included in the vector directsecretion of the transgene product into the egg white. In an alternativeembodiment, the promoter drives expression of the transgene in the liverand signal sequences included in the vector direct the secretion anduptake of the transgene product into the egg yolk. In this embodiment,the promoter is chosen from the group consisting of vitellogenin andapolipoprotein A promoters, and combinations thereof. Preferredenhancers are viral enhancers including, but not limited to, the SV40enhancer, or portion thereof. Lysozyme enhancers may also be employed inaddition to synthetic DNAs thought to bind transcription factors, suchas a steroid hormone response element, e.g., the tandem EREs describedherein.

In one embodiment of the present invention, shown in FIG. 2, controlelements which flank the gene of interest include the SV40 enhancer,three tandem estrogen response elements (ERE), 1.3 kb of the ovalbuminpromoter (5′ flank), 77 bp of 5′ untranslated region (UTR), theN-terminal signal peptide sequence from the chicken lysozyme gene, andthe polyadenylation and termination signals from the SV40 small Tantigen. Sequence Listing 1 sets forth the nucleotide sequence of thepreferred construct. In a preferred embodiment of the present invention,this construct is contained on a 5 kb Xba I fragment which is insertedinto a replication-defective retroviral vector for transgenesis. Thepreferred proviral vector is a derivative of plasmid pSW272. Emerman,M., et al., Cell 39:459-467 (1984); U.S. Pat. No. 4,650,764. Asdescribed in U.S. Pat. No. 4,650,764, herein incorporated by reference,cell lines have been constructed to complement these vectors and producethe viral proteins necessary to package replication-defective retroviralvectors. The packaged vector may infect a cell once, but is incapableitself of subsequent rounds of infection.

The vectors of the present invention are particularly useful inproducing transgenic avians, particularly chickens, by methods known tothose skilled in the art. For example, as described in U.S. Pat. No.5,162,215, herein incorporated by reference, the vectors may be used tointroduce a nucleic acid sequence, e.g., a gene, into cells of an embryoof a chicken. In one embodiment, the vector is microinjected in a newlylaid chicken egg arrested at stage X (not generally more than seven daysold, unincubated), in close proximity to, e.g., directly underneath, theblastoderm. More specifically, an opening about 5 mm in diameter is madein the side of the egg, normally by the use of a drilling tool fittedwith an abrasive rotating tip which can drill a hole in the eggshellwithout damaging the underlying shell membrane. The membrane is then cutout by use of a scalpel or 18 gauge needle and thumb forceps, so that aportion of the shell and membrane is removed thereby exposing theembryo. The embryo is visualized by eye or with an optical dissectingmicroscope with a 6×-50× magnification. A solution, usually tissueculture medium, containing the vector of the present invention, ismicroinjected into an area beneath and around the blastoderm, using amicro-manipulator and a very small diameter needle, preferably glass,40-60 μM outer diameter at the tip, 1 mm outer diameter along it'slength. The volume of solution for microinjection is preferably 5-20 μl.After microinjection, the egg is sealed with shell membrane and asealing material, preferably glue or paraffin. The sealed egg is thenincubated at approximately 38° C. (99.5° F.) for various time periods upto and including the time of hatching to allow normal embryo growth anddevelopment. DNA from embryos and from newly hatched chicks is testedfor the presence of sequences from the microinjected vector. Thepresence of the inserted sequences is detected by means known in the artand appropriate to the detection of the specific gene or if desirable,gene product if the gene or gene product, i.e., protein, is present,eggs from the transgenic chicken are collected and the protein isolated.

In another embodiment, the vector or transfected cells producing thevirus containing the transgene is injected into developing oocytes invivo, for example, as described in Shuman and Shoffner, Poultry Science65:437-1444 (1986), herein incorporated by reference. The same steps ofincubation, hatching, etc. are followed.

As referred to herein, by the term “gene” or “transgene” is meant anucleic acid, either naturally occurring or synthetic, which encodes aprotein product. The term “nucleic acid” is intended to mean naturaland/or synthetic linear, circular and sequential arrays of nucleotidesand nucleosides, e.g., cDNA, genomic DNA (gDNA), mRNA, and RNA,oligonucleotides, oligonucleosides, and derivatives thereof. The phrase“operatively-linked” is intended to mean attached in a manner whichallows for transgene transcription. The term “encoding” is intended tomean that the subject nucleic acid may be transcribed and translatedinto either the desired polypeptide or the subject protein in anappropriate expression system, e.g., when the subject nucleic acid islinked to appropriate control sequences such as promoter and enhancerelements in a suitable vector (e.g., an expression vector) and when thevector is introduced into an appropriate system or cell. As used herein,“polypeptide” refers to an amino acid sequence which comprises bothfull-length protein and fragments thereof.

The term “replication-defective retroviral vector” refers to a vectorcomprising a portion of a retroviral genome capable of infecting a cellbut incapable of unrestricted replication, i.e., multiple rounds ofinfection, usually due to mutations or deletions in the virus genome.The term “REV-derived replication-defective vector” refers to areticuloendotheliosis viral vector that is incapable of unrestrictedreplication.

The term “avian species” includes, without limitation, chicken, quail,turkey, duck and other fowl. The term “hen” includes all females of theavian species. A “transgenic avian” generally refers to an avian thathas had a heterologous DNA sequence, or one or more additional DNAsequences normally endogenous to the avian (collectively referred toherein as “transgenes”) chromosomally integrated into the germ cells ofthe avian. As a result of such transfer and integration, the transferredsequence may be transmitted through germ cells to the offspring of atransgenic avian. The transgenic avian (including its progeny) will alsohave the transgene fortuitously integrated into the chromosomes ofsomatic cells.

In order to more fully demonstrate the advantages arising from thepresent invention, the following examples are set forth. It is to beunderstood that the following is by way of example only and is notintended as a limitation on the scope of the invention.

SPECIFIC EXAMPLE 1 Vector Construction Discussion

Promoter. The protein ovalbumin is the most abundant protein in eggwhite. Ovalbumin is synthesized in the tubular gland cells of theoviduct magnum and secreted directly into the lumen, where it joins theforming egg. The ovalbumin promoter is a well characterized and complexpromoter. Houdebine, L. M., J. Biotech 34:269-287 (1994). The ovalbuminpromoter is regulated by all known classes of steroid hormones (Gaub, M.P., et al., Cell 63:1267-1276 (1990)), and at least eight differentregulatory proteins or groups of proteins are thought to bind to aregion spanning 1.1 kb 5′ to the cap site. These proteins include theTATA binding protein complex (TFIID), the estrogen receptor, activatorprotein 1 (AP-1), which includes the fos and jun gene products andrelated peptides (Curran, T., et al., Cell 55:395-397 (1988)), thechicken ovalbumin upstream promoter transcription factor (COUP-TF)(Wang, L., et al., Nature 340:163-166 (1989)) and an associated proteinS300-II (Sagami, I., et al., Mol. Cell. Biol. 6(12):4259-4267 (1986)), aNF-νB-like nuclear protein (Schweers, L., et al., J. Biol. Chem.266(16):10490-10497 (1991)), and a nuclear factor I (NF-I) homolog.Bradshaw, M. S., et al., J. Biol. Chem. 263(17):8485-8490 (1988). Thecis acting sequences responsible for these interactions are included inthe 1.3 kb fragment used as the preferred promoter in the presentinvention. Although the natural system of ovalbumin expression wasmimicked as closely as possible in the vectors and methods of thepresent invention, the ovalbumin 5′ regulatory region spans some 8 kb(Gaub, M. P., et al., Cell 63:1267-1276 (1990)), which, together withthe other downstream elements (LeMeur, M. A., et al., EMBO Journal3(12):2779-2786 (1994)), is too large for a replication-defectiveretroviral vector. Emerman, M., et al., Cell 39:459-467 (1984). Thus,the 1.3 kb fragment was used. However, it will be appreciated by thoseskilled in the art, that the ovalbumin promoter may include any portionof the ovalbumin transcription unit capable of driving expression of atransgene in the oviduct. Moreover, although the ovalbumin promoter isdiscussed in detail herein, it will be appreciated that other promotersthat drive expression in cells generating the egg white may be employed,including but not limited to, lysozyme, conalbumin and ovomucoidpromoters, and combinations thereof.

In an alternative embodiment, a promoter which drives expression of thetransgene in the liver is employed, such as the vitellogenin orapolipoprotein A promoter, and combinations thereof. Althoughvitellogenin and apolipoprotein A are very abundant proteins in theyolk, they are synthesized in the liver and are then transported to theyolk through the blood. It is deposited in the yolk via a specificreceptor which recognizes an N-proximal fragment of the vitellogeninprecursor. Thus, the vectors of the present invention, when containingthe vitellogenin or apolipoprotein A promoters (or combinationsthereof), they also contain a signal sequence or separate sequencesdirecting the secretion and uptake of the protein in the yolk. Althougha blood-borne intermediate step is required, this type of vector isuseful particularly for antibody production or compounds found in bloodof other species.

Enhancer. The SV40 enhancer has been previously used to increaseexpression from the ovalbumin promoter. Dierich, A., et al., EMBOJournal 6 (8):2305-2312 (1987). AP-1 has been shown to act on theproximal portion of the ovalbumin promoter, and the SV40 enhancer mayincrease the local concentration of the AP-1 complex or some of itscomponents. Curran, T., et al., Cell 55:395-397 (1988). There are othercontrol elements found in the ovalbumin 5′ flank which are not includedin the 1.3 kb ovalbumin promoter. Kaye et al., EMBO Journal 5(2):277-285(1986), discovered four hormone dependent DNAase I hypersensitive sitesin the 5′ flank of ovalbumin chromatin which are correlated withexpression of the ovalbumin gene. Two sites are contained within thepreferred promoter used herein, and the other two lie 3.3 kb and 6 kb 5′to the cap site (sites III and IV respectively). Site III, at −3.3 kb iscontained on a 675 bp Pst I-Xba I fragment from approximately 3.7 kb to3.1 kb 5′ to the cap site. Within this fragment are four halfpalindromic estrogen response elements (EREs) which enhance expressionfrom the ovalbumin promoter in a synergistic fashion. Kato, S., et al.,Cell 68:731-742 (1992). The half EREs are spaced more than 100 basepairs apart from each other. Nonetheless, fusion and deletion studieshave shown both the functionality and necessity of these elements inconferring estrogen responsiveness to a truncated ovalbumin promoter.Kato, S., et al., Cell 68:731-742 (1992). It is thought that severalweakly bound estrogen receptors interact synergistically at this locusto result in more stable receptor-DNA complexes, which then eitherdestabilize the helix, or increase the local concentration oftranscription factors in the vicinity of the promoter.

This region III fragment is not included in the preferred vector of thepresent invention, but instead is replaced by a syntheticoligonucleotide containing a full palindromic ERE adjacent and 5′ to asingle ERE. The estrogen receptor binds palindromic EREs as a dimer withmuch greater affinity than to a single half site. The tandem arrangementof palindromic ERE and a single ERE spaced seven base pairs away addseven further stability. Klein-Hitapaβ, L., et al., J. Mol. Biol.201:537-544 (1988). It is thought that this oligonucleotide functionallyreplaces the −3.3 kb hypersensitive site in vivo.

It is likely that the tandem EREs have a positive effect on geneexpression. EREs have been shown to enhance expression in estrogenresponsive cells, and with promoters containing imperfect EREs. Tsai, S.Y., et al., Cell 57:443-448 (1989); Ponglikitmongkol, M., et al., EMBOJournal 9(7):2221-2231 (1990). There are imperfect EREs in the ovalbuminpromoter, and it is likely that a synergism occurs between the syntheticperfect consensus EREs and the natural ones.

The hormone dependent DNAase I hypersensitive site at −6 kb is containedwithin 1.2 kb. Fusion studies with this DNA fragment show no evidence ofestrogen responsive enhancement of the ovalbumin promoter. Kato, S., etal., Cell 68:731-742 (1992). For this reason, no part or analog wasincluded in the vector shown in FIG. 2.

Previous investigators have demonstrated an absolute requirement for anintracellular phosphorylation cascade; via somatomedin, insulin, or cAMPfor induction of the ovalbumin gene in response to estrogen. Evans, M.I., et al., Cell 25:187-193 (1981); Evans, M. I., et al., Endocrinology115(1):368-377 (1984). Although these studies are more than ten yearsold and the intracellular second messenger cascade mechanisms are nowunderstood in greater detail, the exact mechanism with respect tospecific cis acting sequences in the ovalbumin promoter has not beendemonstrated definitely. It is not unreasonable to suggest, however,that the mechanism involves AP-1 binding, the cis acting sequence ofwhich is included in both the preferred ovalbumin promoter and the SV40enhancer. Curran, T., et al., Cell 55:395-397 (1988).

5′ Untranslated Region. The 5′ untranslated region (UTR) is that ofovalbumin RNA. The ovalbumin gene contains a 5′ leader exon that isspliced to the first coding exon to generate an untranslated region 65bases in length. O'Hare, K., et al., Nucleic Acids Research 7(2):321-334(1979). The vector UTR sequence is copied almost exactly off the cDNA toyield a 5′. UTR that very closely resembles that of ovalbumin RNA. Theonly difference is a one base mutation near the 5′ end which wasnecessary for construction, and an additional 3′ linker, resulting in aUTR 77 bp in length. A 77 base leader is more consistent with Kozak'sstudy which suggest that a minimum of 77 bases is required for maximumtranslational efficiency (Kozak, M., et al., J. Cell Biol.115(4):887-903 (1991)), however, any UTR with a functional sequencearound the start codon may be used.

Signal Sequence. The signal peptide is responsible for transport of theprotein out of the cell, and signal peptide sequence theory is welldeveloped. von Heijne, G., Eur. J. Biochem. 133:17-21 (1983); vonHeijne, G., J. Mol. Bio. 173:243-251 (1984); and von Heijne, G., J. Mol.Biol. 184:99-105 (1985). In the majority of secreted proteins, thesequence is at the N-terminus of the nascent protein and is cleavedduring synthesis and translocation into the endoplasmic reticulum. Inthe case of ovalbumin, however, the sequence is internal to the proteinand is not cleaved (Robinson, A., et al., FEBS 203(2):243-246 (1986)),thus rendering it inappropriate for use in an expression vector. Thesignal sequence of egg white lysozyme was used in the vectors of thepresent invention as a translocation signal because it is a cleavedN-terminal sequence, it functions in vivo in the chicken oviduct, andwill release a protein with a native N-terminus in Saccharomyces.Jigami, Y., et al., Gene 43:273-279 (1986). However, it will beappreciated by those skilled in the art that any signal sequence(s) maybe used.

Gene. The β-galactosidase gene was utilized in the vector set forth inFIG. 2 for two reasons. First, at 3 kb, it is the largest of theavailable reporter genes; many genes encoding commercially valuableproteins are much smaller than this. Thus, if this system can expressβ-galactosidase into the egg, then other genes will likewise beexpressed. Second, β-galactosidase expression can be easily assayed,which facilitates screening of eggs produced from the transgenic hens ofthe present invention. It will be appreciated that any transgene(s) orfragment thereof, may be employed.

3′ Control. Since the transgenic vector of the present invention is aretrovirus, the genome is RNA and, a transcription termination signal inthe orientation of genome synthesis could prematurely stop synthesis andresult in low titers of retrovirus. Other investigators have usedtermination and polyadenylation signals and found relatively littleeffect. Bradyopadhy, P. K., et al., Mol. Cell. Biol. 4(4):749-754(1984). A transcription termination signal should not disrupt genomesynthesis if placed in the opposite orientation, however, but may notbenefit from the enhancing effect of a more proximal LTR in theretroviral vector. Therefore, both orientations of the expression vectorwith respect to the retroviral vector were constructed. Standard stopcodons and the proven polyadenylation signal from the SV40 small Tantigen are included 3′ to the structural gene.

Materials and Methods

Introduction. The β-galactosidase gene together with transcriptiontermination signals and the polyadenylation signal from the SV40 small Tantigen are contained on a 3.5 kb Cla I-Xba I fragment of the expressionvector pSVβ-galactosidase, purchased from Promega Inc. The ovalbuminpromoter is contained on a 1.7 kb Pst I Eco RI fragment of the plasmidpOV1.7 (sequence in Helig, R., et al., J. Mol. Biol. 156:1-19 (1982),Genback accession # J00895 M24999). The SV40 enhancer is contained on a247 bp Nco I-Eco RI fragment of the plasmid pCAT-enhancer, purchasedfrom Promega Inc. All other DNAs in the construct were synthesized denovo. FIG. 3 is a schematic illustrating the construction of theretroviral and expression vectors.

Construction of intermediate #1; pOVSV. The plasmid pOV1.7 contains aHind III site in the first intron of the ovalbumin gene, and a Pst Isite 1.37 kb 5′ to the cap site (see FIG. 4). This 1.6 kb Pst I Hind IIIfragment of pOV1.7 was joined to the Hind III and Nsi I sites ofpSVβ-galactosidase, (Nsi I has compatible ends with Pst I), resulting ina plasmid called pOVSV, shown in FIG. 4. pOVSV is the first of 8intermediates generated to construct the most complex version of thevector.

Construction of intermediate #2; pSigI. A synthetic linker containingthe nucleotide sequence encoding the signal peptide from chickenlysozyme was inserted into the Ssp BI and Cla I sites of pOVSV as shownin FIG. 5. The resulting plasmid is called pSigI. The nucleotidesequence is included in FIG. 5, along with the amino acid sequence ofthe signal peptide and the start codon.

Construction of intermediate #3; pSigPCR. The plasmid pSig I containsundesirable deletions in the 3′ end of the ovalbumin promoter and in the5′ end of the β-galactosidase gene. The β-galactosidase gene wasrestored using PCR. A 3′ primer was used that hybridizes 35 bp 3′ to aunique Sac I site within the gene. Its sequence and the design of thePCR are shown in FIG. 6. The 5′ primer hybridizes to the 5′ end of theβ-galactosidase gene and contains a 17 base 5′ overhang containing aunique Csp 45 I site and eight 5′ nucleotides. Csp 45 I digestiongenerates end compatible with Cla I digestion. PCR was performed for 30cycles, and the products were digested with Sac I and Csp 45 and thenpurified on a low-melt gel. This 1.9 kb fragment was ligated into theunique Cla I and Sac I sites of pSig 1, restoring the β-galactosidasegene and putting it directly 3′ to and in frame with the signal sequencecodons (see FIG. 6). This plasmid is called pSigPCR, and was verified byPvu I digestion, and subsequent sequence analysis.

Construction of intermediate #4; pUTR. A synthetic oligonucleotideencoding the 5′ UTR of ovalbumin was ligated into the Bgl II Ssp BIsites of pSigPCR. This oligonucleotide also contains an Acc 65 I sitenear its 5′ end (centered around the cap site) to allow restoration ofthe promoter in the following steps (see FIG. 7). Proper constructs wereverified by Kpn I digestion. This plasmid is called pUTR, and containsall necessary elements 3′ to the cap site.

Construction of intermediate #5; pUTRAN. The promoter was restored byligating a 1.4 kb Ssp BI partial-Nco I restriction fragment containingthe entire intact promoter from pOVSV into the Nco I and Acc 65 I sitesof pUTR, shown in FIG. 8. Proper recombinants were verified by Bgl IISsp B double digestion. This plasmid, pUTRAN, has a 1.3 kb ovalbuminpromoter driving all necessary downstream elements of the construct.

Construction of intermediate #6; pERE 1. The tandem estrogen responseelements (ERE) are contained on a synthetic oligonucleotide. Because theinvested repeats contained within the EREs form stem loop structureswhich prevent annealing into a double stranded structure, theoligonucleotide was inserted in two steps. The first oligonucleotidecontains two EREs in the same orientation, separated by unique Hind IIIand Spe I sites. This oligonucleotide was ligated into the unique Nsi Iand Nco I sites of pUTRAN, forming the plasmid pERE 1. pERE 1 alsocontains Acc III and Nco I sites useful for insertion of the SV40enhancer, and a terminal Xba I site to allow insertion of subsequentconstructs into the unique Xba I site of the retroviral vector. Properrecombinants were verified by Xba I digestion and sequence analysis

Construction of intermediate #7; pERE. A full palindromic ERE wascreated by ligation of a synthetic oligonucleotide containing the 3′half site into the unique Hind III and Spe I sites of pERE. Theresulting plasmid, pERE, contains a full palindromic ERE and a singleERE half site spaced 7 base pairs away (see FIGS. 9 and 10). Properrecombinants were verified by Hind III Bgl II double digestion, sinceligation of the second oligonucleotide obliterates the unique Hind IIIsite.

Construction of intermediate #8; pUCERE. The plasmid pERE contains allelements of the expression vector except the SV40 enhancer. The SV40enhancer is contained on a 247 bp Eco RI Nco I fragment of the plasmidpCAT-enhancer, available from Promega. pERE contains 3 Eco RI sites and2 Nco I sites, necessitating its subcloning into a vector which lacksthese sites.

The plasmid pUC18 contains only one Eco RI sites and lacks an Nco I sitealtogether. pUC18 was digested with Eco RI and Bam HI (both in themultiple cloning site), blunted with Klenow polymerase, and autoligated.Proper deletions were verified by Eco RI-Sca I double digestion. Themodified vector is called pUCΔBE, and contains a unique Xba I siteuseful in subcloning the construct. Subsequently, the 5 kb Xba Ifragment of pERE, containing the construct, was ligated into themodified vector at that site. This plasmid is called pUCERE.

Construction of pWM0. pUCERE contains Nco I and Eco RI sites 5′ to theEREs, and 3′ to the Xba I site necessary for subcloning into theretroviral vector. It also contains an extra Eco RI site within theβ-galactosidase gene, which necessitates a partial digestion strategy.pUCERE was partially digested with Eco RI, and the linear band isolated.This DNA was digested with Nco I, and the 8 kb fragment recovered from alow-melt gel. The 247 bp Eco RI-Nco I fragment of pCAT-enhancer wasisolated by standard means, and ligated to the pUCERE preparation.Proper recombinants were verified by Xba I-Bgl II double digestion. Thisplasmid is called pWV0, and contains all elements of the transgene on a5 kb Xba I fragment.

Construction of pBCWM. pWM0 and pSW272 both confer ampicillin resistanceto their hosts. To reduce the level of background plasmid whensubcloning into the ampicillin resistant REV vectors, the 5 kb insert ofpWM0 was cloned into the unique Xba I site of pBCSK+, purchased fromStratagene (La Jolla, Calif.), which confers chloramphenicol resistanceto its host. pWM0 was digested with Xba I, and the 5 kb fragmentisolated from a low melt gel. pBCSK+ was digested with Xba I,dephosphorylated, and purified on agarose. The vector and insertfragments were ligated together, and proper recombinants were verifiedby Xba I digest on cultures grown from colonies recovered from LBchloramphenicol (34 μg/ml) plates. This plasmid contains the entireexpression vector on a background of chloramphenicol resistance, readyfor insertion into the replication defective retroviral vector.

SPECIFIC EXAMPLE 2 Retroviral Vector Design and Construction RetroviralVector Design

The plasmid pSW272 (Emerman and Temin, Cell 39:459-467 (1984)) containsa deletion mutant of spleen necrosis virus (SNV), now thereticuloendotheliosis virus (REV). The provirus within the plasmidcomprises the LTRs, the packaging sequence and the thymidine kinase geneand its promoter as a selectable marker for determination of viraltiter. There is a unique Xba I site 5′ to the thymidine kinase promoter.In a previous study, the neomycin phosphotransferase gene had beeninserted in this vicinity (in a Hind III site) resulting in a secondconstruct pME III (Emerman and Temin, Cell 39:459-467 (1984)) which wasused successfully to generate a transgenic chicken (Bosselman, et al.,Science 243:533-535 (1989)). In the same study, the gene encodingchicken growth hormone was cloned into pSW272, and resulting transgenicchickens had significantly higher levels of circulating growth hormonethan nontransgenic controls. pSW272 was modified to better serve as avehicle for the expression vector.

The goals in modifying the retroviral architecture include: replacementof the thymidine kinase gene with the gene conferring resistance tohygromycin B (this eliminates the need for co-transfection, however, italso requires remodeling the ends of the hygromycin gene); eliminationof the 5′ promoter driving the hygromycin gene, and its polyadenylationsignal (this will provide a more stable architecture); provide an Xba Isite 3′ to the hygromycin gene for insertion of the expression vector.

pSW272 was reconstructed in three phases. The first phase was thedeletion of the herpes virus thymidine kinase promoter and structuralgene, and replacement with a synthetic linker. This linker containssites necessary for subsequent manipulations, including a unique Xba Isite at the 3′ end which allows insertion of the 5 kb expressionconstruct contained in pBCWM. The second phase was the introduction oflinkers at the 5′ and 3′ ends of the hygromycin resistance gene thatallow for more specific construction of control sequences at the ends ofthe gene. The third phase tested 4 different arrangements of controlsequences for their ability to stably transfect the C3 cell line. Thearrangement with the fewest control sequences that can stably transfectthe C3 cells was chosen as a preferred proviral vector.

The retroviral vector pSW272 contains the reticuloendotheliosis virus(REV) long terminal repeats (LTR)s, and also the selectable markerthymidine kinase (TK) driven by its own promoter. The LTRs lie at theends of the provirus, and can function as promoters.

By itself, pSW272 is a stable architecture, stable in this casereferring to the ability to generate full length retrovirus with nointernal deletions from its genome. The selectable marker is useful fortitering retrovirus on TK⁻ cells, but not helpful for transfecting thehelper cells necessary to generate the retrovirus.

Additionally, problems may arise when the expression vector is insertedinto pSW272, and the entire construct is then transfected into the C3cell line. The structure of the construct then includes two internalpromoters. The 5′ or left promoter (in this case the ovalbumin promoter)may be unstable in this environment, meaning retrovirus produced fromcells transfected with such a construct experience frequent deletions inthis region (Emerman and Temin, J. Virology 50(1):42-49 (1984)).

The same study provided evidence that a structural gene alone in thatlocation is stable and can be expressed by the LTR, eliminating the needfor a promoter. Since that gene can be virtually any structural gene, itcan be the selectable marker. The C3 packaging cell line containsendogenous Tk activity and consequently it must be co-transfected with aplasmid conferring hygromycin B resistance. The gene encoding hygromycinB phosphotransferase was cloned into the retroviral vector to generatean improved architecture.

The expression vector was then inserted at the unique Xba I site,resulting in a stable architecture, elimination of the need forco-transfection and still enabling the titration of virus on CEF cells.

Retroviral Vector Construction

Construction of pREVΔ schematic illustrating the construction of theretroviral vector is set forth in FIG. 3. The promoter and structuralgene of thymidine kinase are carried on a 2 kb Xba I-Xma I fragment,both of which are unique in pSW272. pSW272 was digested with Xba I andXma I, and the larger fragment (approximately 7 kb) recovered from a lowmelt get. This fragment was ligated to a synthetic oligonucleotidecontaining 5 restriction sites. The resulting construction, pREVΔ, wasconfirmed by Cla I digestion, as Cla I recognizes a site in thesynthetic linker, and a second site outside of the proviral DNA.

Modification of Hygromycin B Phosphotransferase Gene

Modification to pREP4. The hygromycin B phosphotransferase gene iscontained on the plasmid pREP-4, purchased from Invitrogen. However,there are problems with the hygromycin B phosphotransferase gene at boththe 5′ and 3′ ends. At the 5′ end, there is an unfavorable sequencesurrounding the start codon, specifically a second out-of-frame startcodon 4 bp upstream. The 3′ end contains both a polyadenylation signal,which may interfere with retroviral titer, and the LTR from Raus SarcomaVirus (RSV), which must be removed. The 3′ end also lacks convenientrestriction sites necessary to generate the desired constructs, and withthe remodeling, these sites are included.

The plasmids are named after the nature of their control signals. Forexample, the construct containing both a promoter and a polyadenylationsignal is designated p++. Similarly, the plasmid containing a promoter,but no polyadenylation signal is called p+−.

Construction of p++. The 3′ end of the hygromycin B phosphotransferasegene was modified using synthetic double stranded oligos. FIG. 12 showsthe modification of the 3′ end of the Hyg gene. A unique Sca I sitelocated 60 bp from the stop codon, within the gene (see FIG. 12). Asynthetic oligonucleotide containing a Sca I site, the C-terminal codonsand stop codon of the hygromycin marker gene, a Hind III site andflanking Nsi I and Sal I ends was cloned into the Nsi I and Sal I siteson pREP4. Proper recombinants were verified by HindIII-Nru I doubledigestion, and are called p++.

Construction of p+−. p++ was partially digested with Sca I and the 6.3kb fragment recovered from a low melt gel and autoligated, resulting ina hygromycin gene construct lacking 3′ control elements except the stopcodons (see FIG. 12). Proper recombinants were verified by Sca 1-Cla Idouble digestion, and called p+−.

Construction of p−+ and p−−. The N-terminal codons were modified in asimilar manner. Afl III is unique in p++, and lies just 5′ to the startcodon of the hygromycin B phosphotransferase gene. An Aat II site lies25 bases into the hygromycin gene, making an Afl III-Aat II doubledigestion convenient for removal of the promoter. Aat II is not uniqueto p++, and thus a two enzyme strategy was used. p++was digested withCla I and Aat II and in a separate reaction Cla I and Afl Ill. Thedigestion products were run on a low melt gel, and the 5.5 kb Cla I AatII product recovered, and the 2.4 kb Cla I Afl III product recovered.These two DNAs were ligated with a synthetic oligonucleotide (see FIG.13) resulting in p−+. Proper recombinants were verified by Bcl I Alw NIdouble digestion and sequence analysis. The plasmid p+− was treated thesame way, resulting in the plasmid p−−.

The manipulation to the N-terminal portion of the gene was doneindependently for p++, and p+−. The resulting four constructs containall the permutations for the control signals as: 1) with promoter, withpoly A signal—contained on an Nru 1-Hind III fragment of p++; 2) withpromoter, without poly signal—contained on an Nru I-Hind III fragment ofp+−; 3) without promoter, with poly A signal—contained on a Bcl I-HindIII fragment of p−+; and 4) without promoter, without poly Asignal—contained on an Bcl I-Hind III fragment of p−−.

The fragments with promoters (1 and 2 above) were cloned into pREVΔ atthe Sma I (Xma I) and Hind III sites in the multiple cloning site. Sincerecircularization secondary to incomplete digestion is always a concern,the plasmid pREVΔ was digested at three sites Sma I, Hind III, and AccIll. Recovery of the 2 fragments of the appropriate size from low meltgels ensured digestion at both sites within the MCS, and when ligated tothe Nru I-Hind III fragments in a 3-molecule ligation, resulted indesired plasmid readily. These plasmids are called p++R and p+−R.

A similar procedure was used to clone the hygromycin Bphosphotransferase gene without a promoter into REV. The hygromycinconstructs (p−+ and p−−) were digested with Bcl I and Hind III, andcloned in a 3 molecule ligation to gel purified Bcl I, Hind III, and AccIII fragments of pREVΔ, and called p−+R and p−−R.

The insertion of the expression vector into these retroviral vectors isas follows. Each retroviral vector contains a unique Xba I site. Theappropriate plasmid was opened with Xba I, dephosphorylated, and gelpurified. pBCWM contains the expression vector on a chloramphicicolresistant plasmid as a 5 kb Xba I fragment pBCWM was digested with Xba Iand the 5 kb fragment recovered from a low melt gel and ligated to theappropriate retroviral vector. Proper recombinants were verified by XbaI digestion, and orientation was checked by Eco RI digestion. Theseplasmids were named as for their retroviral vectors, with the additionof E and the clone number. For instance: p++RE1, with promoter, withpoly adenylation, in REV and with expression vector in orientation 1.

SPECIFIC EXAMPLE 3 Transgenesis Method 1

Each of the two orientation constructs for a given retroviral vector,was transfected into the C3 cell line, and stable clones selected. DNAis isolated from the clones and analyzed for integrated intact proviralDNA by Southern blot. Appropriate clones are propagated and assayed forretrovirus on CEF cells selected for hygromycin resistance. Clonesproducing high titers are used to generate retrovirus, which is furtherconcentrated by filtration and or centrifugation.

When a clone is found producing high titers of intact viral DNA, eggsare injected as described in U.S. Pat. No. 5,162,215 and Bosselman, R.A. et al., Science 243:533-535 (1989), herein incorporated by reference.Newly laid line 0 SPF eggs are obtained from SPAFAS (Preston, Conn.),and maintained at 20° C. on one side for at least 5 hours. The top ofthe egg is prepped with 70% ethanol, and air dried. The shell is thenopened with a dremmel mototool fitted with a steel burr. 15-25microliters of a solution containing retrovirus is microinjected beneaththe blastoderm. The eggs are sealed and incubated in a Humidaireincubator until hatching.

Ten days after hatching, blood is collected from the chicks, and assayedfor the presence of viral DNA in their genomes by Southern blot and PCR.All chickens are grown to maturity, at which time, the eggs of thesechimeric chickens are tested for the presence of β-galactosidase, andthe rooster semen is tested for viral DNA by Southern blot. Semenpositive roosters are used to sire G2 chickens which are trueheterozygous transgenic chickens.

Method 2

The 5 kb insert (the expression vector) of pBCWM was ligated to pREVΔ orp+−, cut with Xba I, dephosphorylated and purified on a low melt gelClones were screened for the insert by Xba I digestion, and orientationwas checked by digestion with Eco RI.

C3 cells were seeded at 2-3×10⁵ cells per well in a 6 well plate (35 mmwell diameter) and grown overnight in DMEM with high glucosesupplemented with L-glutamine, 10 mM HEPES, 7% calf serum, 400 μg/mlG418, 100 μg/ml gentamicin, 5 μg/ml fungizone (amphotericin B), 100units/ml penicillin G, 100 μg/ml streptomycin sulfate, at 37° C. in 10%CO₂. The cells were transfected using lipofectamine (Gibco LifeTechnologies) at a ratio of 1.5 μg DNA to 8 ul lipofectamine, accordingto the manufacturers specifications. After 5 hours the transfectionmedium was aspirated and replaced with 0.5 ml of DMEM with 7% calfserum, and HEPES. The medium was removed after 48 hours of incubationand used for microinjection or concentrated by ultrafiltration 20-foldwith a filter with a 50 kd cut-off and used for microinjection.

Newly laid fertilized SPF white leghorn eggs were obtained from SPAFASand maintained on their side for at least 5 hours. A pentagonal shapedpiece of shell approximately 0.5 cm² was removed intact from thetop-most portion of the egg using a Dremmel mototool fitted with a steelcutter (part 113). The shell membrane was removed with an 18 gaugeneedle. Micropipettes were pulled on a Sufter puller, trimmed with arazor blade, and checked for diameter and tip angle under a microscope.15 to 20 μl of medium was injected into the subgerminal space using aNarishige micromanipulator (model MN-151) and microinjector (modelIM-6). The hole was patched using donor membranes harvested from eggs inthe same lot held briefly in PBS with penicillin G and streptomycinsulfate used at the concentrations stated above. The shell fragment wasreplaced on top of the donor membrane, and air dried for 10 minutes.Duco cement was used to seal the edges, and air dried for at least 30minutes. The eggs were then set in a Humidaire incubator (model 21) andhatched according to manufacturers specifications.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification and following claims.

All patents and other publications cited herein are expresslyincorporated by reference.

Sequence ID No. 1 Xba I Nco I   1 TCTAGACCAT GGAGCGGAGA ATGGGCGGAACTGGGCGGAG TTAGGGGCGG  51 GATGGGCGGA GTTAGGGGCG GGACTATGGT TGCTGACTAATTGAGATGCA 101 TGCTTTGCAT ACTTCTGCCT GCTGGGGAGC CTGGGGACTT TCCACACCTG151 GTTGCTGACT AATTGAGATG CATGCTTTGC ATACTTCTGC CTGCTGGGGA 201GCCTGGGGAC TTTCCACACC CTAACTGACA CACATTCCAC AGCAGATCCC 251 CCGGAATTCGGTCAAGCTGA CCTACTAGTG GTCATCATGC A TTTCATA GGTAGAGATA ACATTTACTGGGAAGCACAT CTATCATCAT AAAAAGCAGG CAAGATTTTC AGACTTTCTT AGTGGCTGAAATAGAAGCAA AAGACGTGAT TAAAAACAAA ATGAAACAAA AAAAATCAGT TGATACCTGTAAGACGTGAT TAAAAACAAA ATGAAACAAA AAAAATCAGT TGATACCTGT GGTGTAGACATCCAGCAAAA AAATATTATT TGCACTACCA TCTTGTCTTA AGTCCTCAGA CTTGGCAAGGAGAATGTAGA TTTCTACAGT ATATATGTTT TCACAAAAGG AAGGAGAGAA ACAAAAGAAAATGGCACTGA CTAAACTTCA GCTAGTGGTA TAGGAAAGTA ATTCTGCTTA ACAGAGATTGCAGTGATCTC TATGTATGTC CTGAAGAATT ATGTTGTACT TTTTTCCCCC ATTTTTAAATCAAACAGTGC TTTACAGAGG TCAGAATGGT TTCTTTACTG TTTGTCAATT CTATTATTTCAATACAGAAC AATAGCTTCT ATAACTGAAA TATATTTGCT ATTGTATATT ATGATTGTCCCTCGAACCAT GAACACTCCT CCAGCTGAAT TTCACAATTC CTCTGTCATC TGCCAGGCCATTAAGTTATT CATGGAAGAT CTTTGAGGAA CACTGCAAGT TCATATCATA AACACATTTGAAATTGAGTA TTGTTTTGCA TTGTATGGAG CTATGTTTTG CTGTATCCTC AGAAAAAAAGTTTGTTATAA AGCATTCACA CCCATAAAAA GATAGATTTA AATATTCCAG CTATAGGAAAGAAAGTGCGT CTGCTCTTCA CTCTAGTCTC AGTTGGCTCC TTCACATGCA TGCTTCTTTATTTCTCCTAT TTTGTCAAGA AAATAATAGG TCACGTCTTG TTCTCACTTA TGTCCTGCCTAGCATGGCTC AGATGCACGT TGTAGATACA AGAAGGATCA AATGAAACAG ACTTCTGGTCTGTTACTACA ACCATAGTAA TAAGCACACT AACTAATAAT TGCTAATTAT GTTTTCCATCTCTAAGGTTC CCACATTTTT CTGTTTTCTT AAAGATCCCA TTATCTGGTT GTAACTGAAGCTCAATGGAA CATGAGCAAT ATTTCCCAGT CTTCTCTCCC ATCCAACAGT CCTGATGGATTAGCAGAACA GGCAGAAAAC ACATTGTTAC CCAGAATTAA AAACTAATAT TTGCTCTCCATTCAATCCAA AATGGACCTA TTGAAACTAA AATCTAACCC AATCCCATTA AATGATTTCTATGGCGTCAA AGGTCAAACT TCTGAAGGGA ACCTGTGGGT GGGTCACAAT TCAGGCTATATATTCCCCAG GGCTCAGCCA GTGTCTGTAC CTACAGCTAG AAAGCTGTAT TGCCTTTAGCACTCAAGCTC AAAAGACAAC TCAGAGTTCA CCTGTACATA CAGCTATGAG GTCTTTGCTAATCTTGGTGC TTTGCTTCCT GCCCCTGGCT GCTCTGGGGA ATAT

1. A replication-defective retroviral vector comprising: a) a transgene;and b) control elements operatively-linked to the transgene and capableof directing synthesis of the transgene product in avian egg white,wherein the control elements comprise a promoter, a 5′ untranslatedregion and a signal sequence.
 2. The vector of claim 1, wherein thereplication-defective retroviral vector is derived from REV-A.
 3. Thevector of claim 1, wherein the promoter is chosen from the groupconsisting of the ovalbumin, lysozyme, conalbumin and ovomucoidpromoters, and combinations thereof.
 4. The vector of claim 1, whereinthe avian is a chicken.
 5. The vector of claim 1, wherein the controlelements further comprise an enhancer.
 6. The vector of claim 3, whereinthe promoter comprises an ovalbumin promoter.
 7. The vector of claim 5,wherein the enhancer comprises a steroid hormone response element. 8.The vector of claim 5, wherein the enhancer is a viral enhancer.
 9. Thevector of claim 8, wherein the enhancer is a portion of SV40.
 10. Areplication-defective retroviral vector comprising: a) a transgene; andb) control elements operatively-linked to the transgene and capable ofdirecting synthesis of the transgene product in avian egg yolk, whereinthe control elements comprise a promoter, a 5′ untranslated region, asignal sequence and an uptake sequence.
 11. The vector of claim 10,wherein the replication-defective retroviral vector is derived fromREV-A.
 12. The vector of claim 10, wherein the promoter is chosen fromthe group consisting of the vitellogenin and apolipoprotein A promoters,and combinations thereof.
 13. The vector of claim 10, wherein the avianis a chicken.
 14. The vector of claim 10, wherein the control elementsfurther comprise an enhancer.
 15. The vector of claim 14, wherein theenhancer comprises a steroid hormone response element.
 16. The vector ofclaim 14, wherein the enhancer is a viral enhancer.
 17. The vector ofclaim 16, wherein the enhancer is a portion of SV40.
 18. A method fortransferring a transgene to an embryonic chicken cell, comprising thestep of introducing a replication-defective retroviral vector into thecell, wherein the vector comprises the transgene and control elementsoperatively-linked thereto and capable of directing the synthesis of thetransgene product in egg white, wherein the control elements comprise apromoter, a 5′ untranslated region and a signal sequence.
 19. The methodof claim 18, wherein the replication-defective retroviral vector isderived from REV-A.
 20. The method of claim 18, wherein the controlelements further comprise an enhancer.